resize and sharpen

[pbandurian]

I tried a search on "resize sharpen" and didn't find anything that directly addressed my question. When I take largish images, of order 5600x3700, and re-size them for projection at our club to 1024x768 bounding size, I get mixed results. If the image has 'simple detail' such as a macro of a flower, them resulting downsized image will typically look satisfactory and will respond well to a bit of sharpening. But if the image has 'complex detail, say a field of flowers, the re-sized image looks soft and does not respond well to simple sharpening. (The sharpening I use is some combination of LCE via unsharp mask with large radius (10-40 pixels) and small amount (5-25%) and bilateral sharpen.) I generally use the Lanczos 8x8 to re-size because, in theory, it should work best and I'm a sucker for theory. But I've tried the other methods provided in PWP5 with not necessarily better results. I really don't know if my problem is in re-sizing or sharpening technique. I recall reading somewhere that one should never size more than 10% at a time. I haven't tried that because it seems impractical to me for the size of change I'm talking about. Especially for lots of images. Has there been a discussion or work-flow presented on this forum that addresses this specific question and if so could someone point it out to me? And, if not, has anyone any suggestions?

Thanks, Peter

[jsachs]

Resizing, especially by large amounts by necessity blurs small details in the original image since each pixel in the result must combine all the information in a range of pixels in the original. This lost detail cannot be recovered by sharpening - the best you can do is to produce a sharp smaller image. Personally, I usually prefer the simple Sharpen or Sharpen More for this task. Resizing by increments is, in my opinion, a worthless technique.

[den]

There are a couple of nuances that you may wish to consider/experiment for your down sized images:
(1) Try adding a Gamma correction to the Resize workflow of 48-bit color images described in this previous message board thread here: http://www.dl-c.com/board/viewtopic.php?f=4&t=240

(2) When sharpening and/or local contrast enhance-ing the down sized image, use an 'active' mask with the default diagonal Brightness Curve applied keeping the mask white amount half to two-thirds of the mask black amount... this often times allows more aggressive adjustments without strong white edge pixels or obvious light halos.

For the following illustration, a 1728x2304 pixel dimensioned, 48-bit color image was Resized using default Bicubic settings to 200x267 pixels [an 8.64 reduction]. The left half has no gamma correction and Sharpen Amount = 65 with no mask. The right half has gamma correction and Sharpen with a mask described in (2) with mask white amount = 40 and mask black amount = 75.... the left/right differences are difficult to see at the required posting jpeg compression but become readily apparant when an AbsoluteDifference comparison is made of the tiff image versions...

zNoGamma_Gamma_400px-1.jpg (48.76 KiB) Viewed 3894 times

[tomczak]

A bit off topic: Den reminded me of another simple technique of keeping white halos at bay when sharpening using USM: composite the original image with the USM-sharpened one in Darken blending mode.